Boronic acid compounds display a variety of pharmaceutically useful biological activities. Shenvi et al., U.S. Pat. No. 4,499,082 discloses that peptide boronic acids are inhibitors of certain proteolytic enzymes. Kettner and Shenvi, U.S. Pat. No. 5,187,157; U.S. Pat. No. 5,242,904; and U.S. Pat. No. 5,250,720, disclose a class of peptide boronic acids that inhibit trypsin-like proteases. Kleetnan et al., U.S. Pat. No. 5,169,841, discloses N-terminally modified peptide boronic acids that inhibit the action of renin. Kinder et al., U.S. Pat. No. 5,106,948, discloses that certain tripeptide boronic acid compounds inhibit the growth of cancer cells.
Adams et al., U.S. Pat. No. 5,780,454, U.S. Pat. No. 6,066,730, U.S. Pat. No. 6,083,903, and U.S. Pat. No. 6,297,217, hereby incorporated by reference in their entirety for their disclosure of boronic ester and acid compounds, disclose peptide boronic ester and acid compounds useful as proteasome inhibitors. The references also describe the use of boronic ester and acid compounds to reduce the rate of muscle protein degradation, to reduce the activity of NF-κB in a cell, to reduce the rate of degradation of p53 protein in a cell, to inhibit cyclin degradation in a cell, to inhibit the growth of a cancer cell, to inhibit antigen presentation in a cell, to inhibit NF-κB dependent cell adhesion, and to inhibit HIV replication. Brand et al., WO 98/35691, discloses that proteasome inhibitors, including boronic acid compounds, are useful for treating infarcts such as those that occur during stroke or myocardial infarction. Elliott et al., WO 99/15183, discloses that proteasome inhibitors are useful for treating inflammatory and autoimmune diseases.
Bortezomib (BTZ) is a boronic acid proteasome inhibitor currently approved by the U.S. Food and Drug Administration for the treatment of multiple myeloma and mantle cell lymphoma. BTZ is administered clinically by either subcutaneous or intravenous injection. While treatment with BTZ can be effective, administration of free BTZ is plagued by high toxicity, non-specific tissue uptake, side effects, and rapid clearance from circulation. BTZ also possesses limited stability (e.g., a limited shelf complicating the therapeutic use of BTZ. Efforts have been made to develop improved BTZ formulations that address these shortcomings.
Liposomes are spherical vesicles made of a lipid bilayer (e.g., a phospholipid bilayer) that are capable of encapsulating hydrophilic drugs in their aqueous core or hydrophobic drugs within their lipid bilayer. Liposomal drugs can provide prolonged systemic circulation time, decreased drug toxicity, and enhanced drug delivery efficacy. For example, liposomal formulations of certain chemotherapeutics, including daunorubicin (sold under the name DAUNOXOME by Gilead Sciences), doxorubicin (sold under the name DOXIL by Ortho Biotech and under the name CAELYX by Schering-Plough), and vincristine (sold under the name MARQIBO by Spectrum Pharmaceuticals), have been approved by the FDA and exhibit prolonged systemic circulation time, decreased drug toxicity, and enhanced drug delivery efficacy relative to alternative formulations of these chemotherapeutics.
Efforts to prepare liposomal formulations of boronic acid and ester compounds, such as BTZ, have thus far suffered from significant drawbacks. Passive entrapment methods and remote loading methods have both been used to prepare liposomal formulations of BTZ. Passive entrapment methods provide a very low encapsulation efficacy, usually lower than 15%, which is problematic. To address this issue, remote loading was developed in which the entrapment of BTZ in pre-formed liposomes is driven by a pH and chemical gradient. This process is complex and requires overnight incubation. In addition, the stability of the resulting liposomes is poor, which can preclude clinical development.
In order to further increase the shelf-life of liposomes, lyophilization has been utilized. However, as both the passive entrapment and remote loading processes entrap the drug in the aqueous core of the liposome, the contents of the liposome tend to leak out during dehydration and rehydration processes, even in the presence of a lyoprotectant.
New formulations of boronic acid active agents, such as BTZ, that exhibit improved properties, such as enhanced stability, are needed to address these shortcomings. The compositions and methods disclosed herein address these and other needs.